1. Field of the Invention
The present invention relates to the photolithography process used in fabricating semiconductor devices. More particularly, the present invention relates to a method of aligning the dies of a wafer in succession with a photomask of photolithography equipment in the process of exposing the dies to an image borne by the photomask.
2. Description of the Related Art
The fabricating of a highly integrated semiconductor device requires forming a plurality of wiring patterns within a small area. The extent to which a large number of wiring patterns can be formed within a given area depends upon available photolithography exposure techniques more than anything else. In photolithography, respective areas of a wafer are exposed in succession to a predetermined pattern. Subsequently, the exposed areas are developed whereupon the wafer is patterned.
The precision of the exposure process must be increased if the current demand for more highly integrated and for higher quality semiconductor devices is to be met. In order to improve the precision of the exposure process, the capability of the exposure equipment to perform a self-analysis of the exposure process and the precision under which the wafer and the exposure equipment are aligned relative to one another must both be improved. In particular, currently available photolithography equipment has an analysis capability that is insufficient for determining whether the exposure process has yielded a semiconductor product of a quality meeting current requirements. Even the use of deep ultraviolet rays (DUV) to enhance the analysis capability of today""s photolithography equipment poses certain limitations. Therefore, a self-aligning patterning method and the like are being explored as means to produce a pattern of a desired size at a precise position on a wafer.
More specifically, fabricating a semiconductor device involves forming a three-dimensional wiring pattern on a wafer. To form such a wiring pattern, several layers of material are deposited sequentially on the wafer, and these layers are patterned and/or processed. The patterning of a layer of material on the wafer is carried out through the execution of numerous exposure processes in which a respective pattern is transferred repeatedly onto the layer at several areas thereof, respectively. In this process, a photomask bearing the pattern is aligned with certain areas of the wafer in succession so that the pattern of the photomask is positioned relative to patterns already transferred to or to be transferred to the layer of material on the wafer.
In the alignment process, an alignment key pattern is formed on the wafer. The alignment key pattern is used as a reference mark during the process of aligning the wafer with the photomask. An overlay key pattern, formed on the photomask, is used to inspect the state of alignment between the photomask and a selected area of the wafer. The inspection process determines whether the overlay key pattern coincides with or otherwise corresponds to the alignment key pattern, i.e., whether a pattern to be transferred to an area on the wafer during the current exposure process is positioned precisely relative to the pattern that was transferred to another area on the wafer during the previous exposure process.
Image recognition and analysis equipment such as a KLA is used to quantify (measure) the state of alignment between the alignment key pattern and the overlay key pattern. Specifically, the KLA produces an image alignment deviation value and issues a signal representative of this value to the stepper of the photolithography equipment. The image alignment deviation value is used by the stepper to correct, if necessary, the state of alignment between the photomask and the wafer.
In a conventional alignment method, the state of alignment is inspected and used to position the wafer in the processes of exposing the individual areas of the wafer. That is, the inspection process is carried out in connection with the exposure of each and every die of each and every wafer. Such a method comprising numerous inspecting steps is a hindrance on the production efficiency of the exposure process and becomes particularly onerous when the wafer comprises a large number of dies.
Recently, therefore, an alignment method has been developed in which several sample dies from a wafer are selected, the state of alignment of only these sample dies is inspected, deviation data is produced from the inspecting of the state of alignment of the sample dies, and a final job file is produced from the deviation data. Basically, the final job file dictates an overall alignment corrected position for wafers loaded in the stepper. That is, once a wafer is loaded onto a stage in the stepper, the wafer is moved linearly or is rotated, or the exposure equipment is focused, on the basis of the final job file, to set the wafer at the corrected position. After the wafer is set at the corrected position, the wafer stage is moved in increments determined by the size of the dies, to execute the exposure processes without any further inspecting of the state of alignment of the individual dies.
Such an overall alignment correction method is advantageous in terms of enhancing the efficiency of the exposure process. Furthermore, under this method, the actual aligned positions of the individual dies does not deviate much from the ideal positions because the photolithography equipment, i.e., the stepper, is in general very precise. At present, however, the so-called process margin of the exposure process has become very small in order to meet the strong demand for more highly integrated semiconductor devices. Therefore, even a small deviation per die between the actual and ideal state of alignment becomes problematic.
Such small deviations are shown in FIG. 1. In this figure, the magnitude and direction of alignment errors or deviations are represented by the vectors. As can be appreciated from FIG. 1, under the conventional overall alignment correction method, most of the dies will have a similar or the same deviation. Therefore, under the overall alignment correction method, the same inferior state of alignment is present throughout a significant part of the wafer.
There are several potential causes for the occurrence of such a constant alignment deviation. One of the causes might be merely the increase in size of the wafers that are being processed today. Also, seeing that many different devices make up the exposure equipment used in fabricating a semiconductor device, some characteristic particular to the device(s) can give rise to an alignment deviation which can not be overcome using the conventional overall alignment correction method. For instance, a typical piece of exposure equipment comprises a flat plate having side walls extending along X-and Y-axes, respectively, and a mirror mounted to one of the side walls. The wafer is mounted on the flat plate, whereby the plate serves as a wafer stage. The mirror constitutes an interferometer that is used to determine the distances between the side walls of the plate and reference positions as measured along the X-and Y-axes, respectively. These distances are used to precisely position the flat plate, on which the wafer is mounted, during the exposure process. However, the mirror is generally not exactly planar, and so the origin of light reflecting from the mirror can not always be exactly determined. Thus, a measurement value obtained by the interferometer to represent the distance along the X-axis between the flat plate and a reference position might not be accurate. In this case, the flat plate might deviate from its desired position when it is moved along the Y-axis as fixed in the direction of the X-axis on the basis of the measurement value produced by the interferometer.
Of course, such a deviation inherent in the exposure equipment could be compensated for to some extent by reflecting its value in an operative program of the equipment. For instance, in the example of an interferometer described above, the interferometer is first calibrated (referred to as a grid calibration) at each of several constant intervals of movement of the flat plate to determine its deviation with respect to an operation employing an ideal (planar) reflective surface. The value of the deviation is then input as basic data to a higher level operative program of the exposure equipment. Consequently, the movement of the flat plate over the predetermined intervals during the exposure processes is supplemented with additional movement in a direction determined based on the deviation value. That is, the operation of the equipment is modified to take into account the inherent characteristics of the equipment that would otherwise affect the precision of the alignment method.
However, in this control method, the deviation values-are not based on measurements at all virtual grid points in the plane of the X- and Y-axes. Rather, the deviation values are obtained, respectively, at every unit length along the X-axis and at every unit length along the Y-axis. These deviation values are referenced to corresponding positions spaced from one another by the unit lengths in the directions of the X- and Y-axes. Therefore, this control method does not take into account all the possible deviations that can occur in the alignment method.
Moreover, the flat plate, i.e., the wafer stage, can not be moved exactly parallel to the X- and Y-axes due to limitations in the mechanical precision under which the flat plate can be guided by guide members. Thus, even if the deviation caused by the reflecting surface of the mirror of the interferometer is a known constant, this deviation might not manifest itself equally at all positions of the wafer stage. In other words, any deviation is not only dependent on the planarity of the reflecting surface of the mirror of the interferometer but also on the position to which the wafer stage has been guided. Thus, the magnitude and direction of the deviation determined from the grid calibration can change during operation.
Considering that a wafer is subjected to tens of the exposure processes in order to fabricate a semiconductor device on the wafer, that the exposure equipment is made up of several different devices, and that each of the devices has its own deviation characteristic, it is difficult to compensate for the deviation per die using the overall alignment correction method. Furthermore, the alignment deviation per die due to inherent characteristics of the device(s) of the exposure equipment becomes more problematic the smaller the process margin becomes. Meanwhile, the other conventional method of inspecting the state of alignment of each of the dies for each and every wafer and correcting the deviations when revealed by the inspecting steps is simply impractical in terms of production efficiency.
Accordingly, an object of the present invention is to provide a method of aligning the dies of wafers with an exposure device, that substantially obviates one or more of the above-described limitations and disadvantages of the prior art.
More specifically, an object of the present invention is to provide an alignment method which compensates for not only the overall alignment deviation that is uniform with respect to several dies of the wafer but also compensates for the alignment deviations that differ from the overall alignment deviation and are unique to the other dies of the wafer.
To achieve this object, the present invention provides a method in which a given number of wafers to be processed is designated as a basic lot unit, and for every basic lot unit, one of the wafers to be processed is selected. Then, states of alignment between all of the dies of the selected wafer and the exposure device are inspected. The results of the inspections are used to produce alignment deviation data, and the data is mapped to produce a map of the respective deviations of the dies from ideal positions, i.e. the positions at which they would be ideally aligned with an exposure device. Such alignment deviation data for each die is transferred to a stepper together with basic data about the wafer, namely, data of the intervals at which the wafer must be moved between exposure processes as dictated by the size and spacing of the dies, etc.
The alignment deviation data can be transferred to the stepper via magnetic diskette, an on-line transmission or the like. The alignment deviation data may be transferred as raw data devoid of concrete variables for programming or may be converted into data capable of being input to an operative program stored in the stepper. That is, before being transferred to the stepper, the raw data can be converted into processed data, capable of being read by the stepper, by image analysis equipment or a specific mapping tool that is auxiliary to the stepper.
In any case, consequently, the stepper produces a preliminary job file from the alignment deviation data, and from basic data about the wafer. The basic data will include the distances between the dies of the wafer as well as the planned sequence in which the dies are to be successively brought into alignment with the exposure device during the exposure process.
If the alignment deviation data was raw data that was transmitted to the stepper, the stepper converts the raw data into the data making up the preliminary job file. On the other hand, if the alignment deviation data had been transmitted to the stepper as data converted into programming language by an auxiliary piece of equipment, the stepper produces its preliminary job file from the programming language. At this time, the preliminary job file establishes the basic setting of the stepper. More specifically, the preliminary job file establishes for the stepper the initial exposure position of the wafer stage in the X- and Y-directions, the depth of focus or magnification to be used in the exposure process, and an increment of angular rotation necessary to correct a deviation of the wafer stage from an ideal angular orientation. In other words, the preliminary job file causes the stepper to execute an overall alignment correction method.
Next, alignment correction data is generated for compensating for the deviation of each die of the wafer. The correction data may be produced by calculating correction values and then deducting these correction values, respectively, from original alignment deviation data values determined by the inspecting of the individual dies of the selected wafer.
The correction data for each die so obtained is inputted into the preliminary job file by a feed-back system. The preliminary job file is thereby modified by the correction data to produce a final job file. The modification of the preliminary job file generally involves using the correction data to change, if necessary, the die-to-die distances that were input to the stepper as part of the basic data of the wafer (the exception being the die-to-die distances between dies at the ends of adjacent rows of the dies-the reason for this will be evident from the detailed description). Thus, according to the final job file, the basic wafer data of the die-to-die distances is adjusted to reflect the correction data so that the wafer stage will be moved in increments effecting a fine control in the positioning, i.e., aligning, of the dies relative to the exposure device. Thus, the increments over which the wafer stage might be moved between successive processes of exposing the dies may be rather different.
Once the final job file is so configured, the stepper moves the dies in succession into alignment with the exposure device, under the control of the final job file, whereupon the dies of the selected wafer are successively exposed to the image of the photomask.
In addition, another object of the present invention is to provide a method, in the mass-production of semiconductor devices, of exposing the dies of a plurality of wafers using photolithography and which method can effect a correction of alignment deviations to a great extent without seriously compromising the efficiency of the process.
Thus, in addition to the steps described above, the method of the present invention also includes steps of sampling the states of alignment between the next wafer(s) of the lot and the exposure device, and of providing a respective preliminary job file and a respective final job file for the next wafer(s) of the lot. If a statistical analysis reveals corresponding deviations between the selected wafer and the next wafer(s) of the lot, the same correction deviation values are used to provide the final job file. Otherwise, new correction deviation values are produced.
That is, alignment deviations may differ among even wafers within a lot processed by the same equipment and under the same conditions. Therefore, the initial state of alignment between only some (several) dies of the next wafer of the lot and the exposure device are inspected. Then, the basic setting of the stepper is established as the result of the inspections. In other words, the preliminary job file is generated independently of that generated for the previous wafer of the lot. However, should an analysis of the states of alignment reveal the same deviations between corresponding dies of the wafers in the lot, then the correction deviation data associated with the corresponding dies is appropriately transferred to the final job file.
In particular, when the method of the present invention is to be practiced, a technician will generally decide the number of wafers that are to make up the lot according to the current state of the manufacturing line and process being carried out thereby. A lot is established on the premise that the wafers having undergone the same processes (type and condition) prior to the exposure process will tend to produce the same alignment deviations. That is, if a die of a wafer of the lot shows an alignment deviation, the corresponding dies of the other wafers of the lot should show the same alignment deviation. The inspecting of the states of alignment of only a sample of the dies of the wafer(s) merely confirms the results of the inspecting of the states of alignment of all of the dies of the sample (first) wafer of the lot.
In addition, the number of wafers that are to constitute a lot can be changed during the course of the overall manufacturing process. For example, the manufacturing process may develop some serious flaw and results may show that deviations in the alignment states are not being sufficiently corrected. In such a case, a mode of operation of the present invention can be changed. That is, the number of wafers constituting a lot can be reduced mid-operation. Conversely, should the results show a strong trend toward uniform alignment state deviations, the number of wafers constituting a lot can be increased mid-operation.